\begin{tikzpicture}[>=Stealth,
    every node/.style={fill=white, inner sep=1pt},
    scale=0.8,
]
    \pgfmathsetmacro{\a}{6.1}
    \pgfmathsetmacro{\b}{1.7}

    \draw [ultra thick, pattern={mylines[angle=45, distance={5pt}]}]
        (0, 0) rectangle (\a, \a);
    \draw [ultra thick, fill=white] (0, 0) rectangle (\b, \b);
    \draw [ultra thick, fill=white] (\a - \b, 0) rectangle (\a, \b);
    \draw [ultra thick, fill=white] (0, \a - \b) rectangle (\b, \a);
    \draw [ultra thick, fill=white] (\a - \b, \a - \b) rectangle (\a, \a);

    \draw [<->] (\a - \b, -0.3) to [xianduan={above=0.3cm}] node {$b$} (\a, -0.3);
    \draw [<->] (0, -0.7) to [xianduan={above=0.7cm}] node {$a$} (\a, -0.7);
\end{tikzpicture}
